This invention relates to a booster circuit for a liquid crystal display device of a timepiece.
A conventional watch having a liquid crystal display device is shown schematically in FIG. 1. Output signals from a quartz crystal oscillator 1 are fed to a driving circuit 2, and a liquid crystal display device 3 is driven by the driving circuit 2 and shows the corresponding time.
Direct current voltages from a direct current source 4 and a DC - DC converter 5 are fed to the driving circuit 2. In the DC - DC converter 5, direct current voltage from the source 4 is changed to a pulse voltage, after it is boosted at some value of voltage, it is changed to direct current voltage by the rectifier circuit.
In a small size watch having a liquid crystal display device, particularly in a wrist watch, it is preferable to incorporate a small size cell having a long life. For satisfying this need, a silver oxide cell having a 1.5 V terminal voltage has been used, however, it is not sufficient to drive a liquid crystal display device directly. Then some kind of booster, such as a DC - DC converter shown in FIG. 1, must be incorporated into a watch together with a cell.
For example, a circuit shown in FIG. 2 is used as a DC - DC converter. It is called a blocking oscillator and consists of a transformer T of which the winding ratio is 1 : 1, a condenser C.sub.1, a resistor R.sub.1 and a transistor Q.sub.1. The DC voltage is changed into a pulse voltage by this blocking oscillator.
A cubic winding T.sub.3 of which the winding ratio is n times that of a secondary winding T.sub.2 is disposed together with the secondary winding T.sub.2. A pulse induced on the second winding T.sub.2 is boosted by the cubic winding T.sub.3 and is rectified by a rectifier circuit having a diode D.sub.1 and a smoothing condenser Co. Then n times the DC voltage of a direct current source 4, for example six volt, appears between output terminals t.sub.1 and t.sub.2.
The construction of the DC - DC converter is such that a transformer having three windings is required. Because of this, it is very difficult to get a small size watch having a liquid crystal display device. Furthermore, since three coils are incorporated into the converter, the connection of terminals is very troublesome, and to make matters worse, conversion efficiency is lowered. Then the cell is exhausted so much earlier.
In an electronic watch, voltage fluctuation of the power source happens to occur by instantaneous disconnection between a cell and a connecting electrode owing to mechanical shock.
If the capacitance of the smoothing condenser Co is relatively small compared with the power consumption of the load resistance (logic circuit), the output voltage of said booster circuit would promptly be dropped. If this voltage drop is to large, the memory of the logic circuit would be lost.
When this voltage is recovered after that, and sufficient power is supplied to the logic circuit, but the memory of said logic circuit is not recovered. Because of this, display time of the watch gets out of order, and adjustment of the display time is requested.